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From my understanding the sort_merge_passes indicates the number of temporary files created during the sort process when the sort_buffer_size is too small to have everything sorted in memory. In your case it appears that everything gets sorted in memory. Decreasing sort_buffer_size may result in this creating temporary files to perform the sorts when there is insufficient sort buffer.

I think we need to take a step back and look at the data that you are trying to work with. How large is your table (how many records)? We will also need an EXPLAIN on your query to see what you are trying to do and how MySQL is handling this. It might be that you have so many rows that a large portion of your time is being used up during the full scan of the table instead of adding appropriate indexes.

If your table is small for example, each row used in the sort contains 20 bytes then 10k of rows will only use up 200k of space. So I am assuming you have far more memory allocated to sort buffer that this is as efficient as you can get it. What sort of timings are you getting on the SQL request and what are you expecting this to be?